1. Field of Invention
The present invention relates to a toner which can be used in an electrophotographic apparatus such as a copier, printer, or facsimile machine, to a method of manufacturing the same, and to an image forming method making use of a toner for electrophotography.
2. Description of Related Art
Conventional systems for fixing a toner for electrophotography include the pressure fixing system in which a pressurizing roller is employed at room temperature, a contact heating fixing system in which a heating roller or the like is used, an oven fixing system involving heating in an oven, a flush fixing system using a xenon lamp of the like, an electromagnetic fixing system making use of microwaves or the like, and a non-contact fixing system such as a solvent fixing system using a solvent steam. Among these, the most commonly employed today are the oven fixing system and the contact heat fixing system because their reliability and safety are relatively high. Especially, the contact heating type of fixing system using a heating roller, a belt, or the like generally comprises a heating roller or a belt with a heat source provided thereon and a pressurizing roller or a belt, and fixing is performed by passing a fixed sheet with a toner image thereon over the heating roller or the belt in a state wherein the surface of the fixed sheet with the toner image thereon is pressed and contacted to the heating roller or the belt. Because the surface of the heating roller or a belt surface directly contacts the toner imager surface of the fixed sheet, the heat efficiency is high and fixing can be performed quickly, making this system especially popular.
In the thermal fixing system, shortening of the time from the time from when a power is turned ON until the temperature of a fixing device rises to a level at which the fixing device can be used for fixing a toner image, in other words shortening of the warm-up time, and fixing under low temperature in order to reduce the energy consumption rate are both desired. In recent devices it is a requirement that, in order to save energy, supply of power to the fixing machine be stopped when the fixing machine is not being used, and therefore a temperature of the fixing device should preferably rise to the fixing temperature immediately after power supply is started, so that the fixing operation should be performed at as low a temperature as possible. By lowering the fixing temperature to the extent possible, it becomes possible to realize a faster printing speed while maintaining the same power consumption rate, and further to prolong the service lifetime of components for fixing such as a heating roller required for the contact heating type of fixing system, which is advantageous also for cost reduction. In the conventional technology, however, when the temperature required for fixing a toner image is lowered, also the glass transition temperature of the toner particles is lowered, which makes it difficult to stably store the toner for a long time. To simultaneously ensure compatibility with low temperature fixing of a toner image at a low temperature and long term, stable storability of the toner, it is necessary to develop a toner having what is known as a “sharp” melting capability, in other words, a toner which has a high glass transition temperature and viscosity of which quickly drops in the high temperature range.
Generally, however, non-crystalline resins are used for production of toners. Because the glass temperature point and molecular weight of such resins vary over a wide range, it is necessary to control the composition and molecular weight of a resin used for production of a toner within extremely narrow ranges for realization of the sharp melting capability. To obtain a resin satisfying the requirements noted above, it is necessary to employ a specific production method and to carefully control the molecular weight of a resin used for production of a toner by processing the resin by means of, for example, chromatography. As a result, the production cost of the resin is higher and unnecessary resins are produced, which is undesirable from the viewpoint of environment protection.
To provide a toner adapted to fixing at a low temperature as described above, the possibilities provided by the method in which a crystalline resin is used as a binding resin have been discussed (Refer to, for instance, Japanese Patent Publication No. SHO 56-13943, Japanese Patent Publication No. SHO 62-39428, Japanese Patent Publication No. SHO 63-25335). In this method, by using a crystalline resin, hardness of the toner is maintained at a temperature lower than the melting point of the crystal, and when the temperature exceeds the melting point the viscosity quickly drops in association of melting of the crystal. Thus, the adaptability to being fixed under a low temperature is realized. However, in the technology disclosed in the above documents, for example, in Japanese Patent Publication No. SHO 56-13943, the melting point of the crystalline resin is in the range from 62 to 66° C., and its melting point is too low, so that the reliability of toner powder and images developed with the toner is rather low. There is a further disadvantageous problem in that the crystalline resin disclosed in Japanese Patent Publication No. SHO 62-39428 and Japanese Patent Publication No. SHO 63-25335 is not sufficiently adapted to being fixed to paper.
Polyester resin is one of the crystalline resins, which may have improved adaptability to being fixed to paper. An example of a toner based on the crystalline polyester resin is disclosed in, for example, Japanese Patent Publication No. SHO 62-39428. This patent publication proposes a method in which non-crystalline polyester having a glass transition temperature of more than 40° C. and crystalline polyester having a melting point in the range from 130 to 200° C. are mixed together in use. Although this method provides resins with excellent adaptability to being powdered and also having the excellent capability for preventing blocking, the crystalline polyester resins provided by this method generally have a high melting point, so that fixing of a toner image can not be performed at a temperature lower than that in the prior art. There is also an example in which a crystalline resin having the melting point of less than 110° C. and a non-crystalline resin are mixed to prepare a toner (as disclosed in, for example, Japanese Patent Publication No. HEI 4-30014). In a case wherein a non-crystalline resin is mixed in a crystalline resin, the melting point of the toner becomes lower, which causes several disadvantageous problems in actual use such as blocking of the toner and poor storability of images developed with the toner. When a content of the non-crystalline resin component is larger, the characteristics of the non-crystalline resin are substantially reflected to the prepared toner, so that it is difficult to perform fixing at a temperature lower than that in the conventional technology. In actual practice, therefore, a single crystalline resin must be used for preparation of a toner or to mix only an extremely small quantity of non-crystalline resin in a crystalline resin for preparation of a toner.
As described above, it is desirable to use, to the extent possible, a single crystalline polyester resin for fixing a toner image with a heating roller. Examples of using a crystalline polyester resin are disclosed in, for example, Japanese Patent Laid-Open Publication No. HEI 4-120554, Japanese Patent Laid-Open Publication No. HEI 4-239021, and Japanese Patent Laid-Open Publication No. HEI 5-165252. However, there are resins in which alkylene alcohol having a small number of carbon atoms or aliphatic alcohol is reacted with carboxyl groups of a telephthalic acid. These patent publications include descriptions concerning crystalline polyester resins, but the described crystalline polyester resins are only partially crystallized, and the viscosity of the toners prepared with those resins does not vary in connection with changes in temperature. Although resistance against blocking and the storability of images developed with such toners are excellent, but fixing with a heating roller cannot be performed at a temperature lower than that in the prior art.
On the other hand, toner particles manufactured by conventional kneading and pulverizing methods generally have heterogeneous shapes and heterogeneous compositions. Although shape and surface composition of toner particles does slightly change according to their adaptability to pulverization, type of pulverizer, and conditions for pulverization, these parameters are not easily controlled in order to produce particles within a desired range. Recently there is also a tendency to produce toners with smaller particle size in order to improve the image quality developed with the toners, but when toner particles are produced with materials adapted to be pulverized for producing toners with smaller particle size, the toner particles may further be pulverized due to a mechanical force such as a shearing force within a developing unit, which may in turn change the form of the toner particles. As a result, for a developer comprising two components, namely toner particles and a carrier, adaptability of the developer to being electrically charged may be degraded quickly due to deposition of the fine particles to a surface of the carrier, and, in a case of a developer comprising one component, namely comprising only toner particles, the toner may easily be scattered due to widening of a range of particle size distribution, or the adaptability to development may be degraded due to change in forms of the toner particles, which may in turn degrade quality of images developed with the toners. As such, there have been technological restrictions in reducing the toner particle size.
Further in a case of preparing a toner by adding a large quantity of a release agent such as a wax in a resin according to the conventional type of kneading and pulverizing method, because the release agent is more fragile than the resin, the release agent may often come out of a surface of the toner particle. It is advantageous to improve the release property during fixing and to clean off a toner transferred from a photoreceptor, but as the release agent on a surface of the toner particle easily transfers to and contaminates such components as a development roller, a photosensitive roller, and a carrier, reliability is lowered.
Further, when forms of the toner particles are not heterogeneous, the fluidity is not sufficient even if a fluidizing agent is added, and further fine particles of the fluidizing agent remove to concave sections of the toner particles and are buried therein, so that the fluidity lowers as use continues, and sometimes the adaptability of the toner to being developed, transferred, and cleaned may be disadvantageously lowered. When the toner particles are heterogeneous, especially the adaptability to being transferred is further degraded because the adhesiveness increases due to such reasons as increase of contact points. If a larger quantity of fluidizing agent is added to prevent the problems as described above, black points may be generated on the photoreceptor, and in a case of the two-component developer, the fluidizing agent is deposited on the carrier, which disadvantageously degrades the adaptability to being electrically charged.
The present invention was made in light of the circumstances described above in order to solve the above problems.
More specifically, it is an object of the present invention to enable fixing at a lower temperature as compared to that in the prior art, substantial reduction of energy consumption in the fixing step, and shortening of the warm-up time by using a crystalline polyester resin having the low melting point as a main component of a binding resin. It is another object of the present invention to provide the excellent storability of images after fixing.
It is still another object of the present invention to provide a toner for electrophotography with superior adaptability and which is transferred by mixing at least a binding resin particle dispersion liquid with a colorant particle dispersion liquid and adding a coagulant in the mixture to prepare toner particles with spherical form.